Chiral tilted smectic liquid-crystalline phases with ferroelectric properties can be prepared by adding a suitable chiral dopant to basic mixtures having one or more tilted smectic phases (L.A. Beresnev et al., Mol. Cryst. Liq. Cryst. 89, 327 (1982); H. R. Brand et al., J. Physique 44, (lett.), L-771 (1983)).
Such phases can be used as dielectrics for fastswitching displays which are based on the principle of SSFLC technology described by Clark and Lagerwall (N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 (1980); U.S. Pat. No. 4,367,924) which is based on the ferroelectric properties of the chiral tilted phase. In said phase the long molecules are arranged in layers, the molecules having an angle of tilt to a line perpendicular to the layer. On proceeding from layer to layer, the direction of tilt changes by a small angle with respect to an axis situated perpendicular to the layers so that a helical structure is formed. In displays which are based on the principle of SSFLC technology the smectic layers are arranged perpendicularly to the plates of the cell. The helical arrangement of the tilt directions of the molecules is suppressed by a very small spacing of the plates (approx. 1-2 .mu.m). As a result the longitudinal axes of the molecules are forced to arrange themselves in a plane parallel to the plates of the cell, as a result of which two distinct tilt orientations are produced. By applying a suitable alternating electric field it is possible to switch back and forth between these two states in the liquid-crystalline phase which exhibits a spontaneous polarization. Said switching process is substantially faster than in the case of conventional twisted cells (TN LCDs) which are based on nematic liquid crystals.
A considerable disadvantage for many applications of the materials at present available with chiral tilted smectic phases (such as, for example, Sc.) is that the dielectric anisotropy has values greater than zero or, if negative, values only slightly different from zero. Negative values of the dielectric anisotropy are necessary if the necessary planar orientation is produced by superimposing an AC holding field with low amplitude on the drive field (J. M. Geary, SID Conference, Orlando/Fla., April/May 1985, Paper 8.3). The use of materials with strongly negative dielectric anisotropy generally leads to a strong decrease in the spontaneous polarization and/or to unfavorable values for pitch and/or tilt. In addition, the temperature range of the ferroelectric phases is generally restricted in an unfavorable manner.